1. Field of the Invention
This invention relates to a method for manufacturing silicon single crystals and relates to wafers adapted for producing semiconductors. In particular, this invention relates to a method for manufacturing CZ silicon single crystals suitable for high-temperature heat treatments of CZ silicon wafers in a hydrogen atmosphere (hereinafter referred as hydrogen heat treatment). Furthermore, this invention relates to wafers that are cut out from single crystals manufactured by the above method and undergo a hydrogen heat-treatment to get rid of defects formed therein.
2. Description of the Related Art
Most substrates used for fabricating semiconductor components are silicon single crystals, which are dominantly manufactured by the CZ method. In the CZ method, polycrystalline silicon is fed into a crucible and is heated to melt into a liquid. Then, a seed crystal disposed in a seed chuck is immersed into the liquid stored in the crucible; subsequently, the seed is rotated and pulled to grow a silicon single crystal of preset diameter and length.
Silicon single crystals acquired through the above process are sliced to obtain wafers, and subsequently devices are mounted on the activated surfaces of the wafers. However, due to the tendency of miniaturization and high-integration of device structures, the voltage endurance characteristic of a gate oxidation layer has become highly important. Japanese Patent Gazette JB HEI 3-80338 disclosed a method for reducing defects contained in the gate oxidation layer during its forming process. The '338 patent suggested heat-treating a silicon wafer in a non-oxidizing atmosphere containing hydrogen gas prior to the process of forming a thermal oxidation thin layer on the surface of the silicon wafer. It has been known that grown-in defects contained in a CZ silicon wafer disappear after a hydrogen heat treatment and the oxidation layer voltage endurance characteristic of the treated CZ silicon is excellent. The above grown-in defects, such as LSTD (Laser Scattering Tomograghy Defects), FPD (Flow Pattern Defects), or COP (Crystal Originated Particle),can be detected near the outer surface layer of a wafer.
However, the distinguishable outcome brought about by hydrogen heat treatment can only be found near the most outer surface layer of the wafer. The inventors of this invention noticed that the smaller the defect size, the better the result of eliminating defects achieved through hydrogen heat treatment. Therefore, a proposal (Japanese Patent Application No. HEI 9-27213) was offered. Namely, the proposal is a method for raising the cooling rate while passing through the defect producing temperature zone in the process of pulling a crystal so as to miniaturize the defect sizes.
However, when a silicon single crystal diameter is over 200 mm, the heat capacity is larger than that of conventional small-diameter silicon single crystals; it is quite difficult to obtain a cooling rate capable of miniaturizing the defect sizes to a significant extent. In other words, due to insufficient cooling in the process of the pulling operation, it is quite difficult to eliminate defects existing in large silicon ingots with the aid of hydrogen heat treatment.